Uveal iridophoroma in a betta fish (Betta splendens)

Chromatophoromas are neoplasms arising from pigment cells in vertebrates. Iridophoromas are a type of chromatophoroma that are reported in several teleost species. There are multiple case reports of this diagnosis in betta fish (Betta splendens), but all previously reported cases originated from the skin. This is the first report of an intra-ocular iridophoroma. An adult betta fish had buphthalmia of the right eye and enucleation was performed. The fish survived surgery but was found dead 2 days later. The eye and entire body were examined histologically, and the right eye was also examined ultrastructurally. Histologically, the uveal tract of the right eye was unilaterally and markedly expanded by a neoplasm that expanded and obliterated the iris and choroid and regionally invaded the cornea and sclera. The neoplasm was composed of spindle cells that contained pale green, birefringent, crystalline granules (iridophores). Transmission electron microscopy revealed that neoplastic cells contained a single elongated nucleus and one or more thin cytoplasmic bundles of reflecting plates that were oriented parallel to the basal lamina, further confirming the diagnosis of iridophoroma. This is the first reported case of an iridophoroma arising from the uveal tract. Most cases of iridophoromas and other chromatophoromas in fish are reported as benign. However, there are no established histological criteria of malignancy in these neoplasms. Despite the bland cellular morphology, most reported cases of iridophoromas and other chromatophoromas in bettas (including this case) had substantial tissue invasion and/or destruction. This suggests that iridophoromas and chromatophoromas in bettas may have local invasion, consistent with malignancy, despite bland cytological features.